The present invention relates to a coating composition for forming thick coating in amount of 1 to 20 kg/m.sup.2.
In general, coating compositions for thick coating comprise a bonding agent, a dispersant or solvent, an aggregate, a pigment and other adjuvants.
As the bonding agent, there have heretofore been used cements, organic resins, water glasses, silica sols or mixtures thereof. When water glass is used as a bonding agent for a coating composition of this type, efflorescence and reduction of the water resistance are brought about based on the strong alkali of the water glass. Accordingly, use of water glass is not preferred. On the other hand, use of an organic resin as the bonding agent is not preferred from the practical viewpoint because it is expensive, the surface hardness is low in the resulting cured coating and reduction of the water resistance is brought about in some cases. Moreover, in case a silica sol bonding agent is employed, though such advantages as prevention of occurrence of efflorescence and improvement of the water resistance can be attained, no good adhesion is obtained in the coating composition and properties of the coating composition are degraded when it is stored for a long time. Accordingly, use of a silica sol is also not preferred from the practical viewpoint.
Various composite coating liquids have heretofore been proposed as coating liquids to overcome the foregoing defects. For instance, a composite coating liquid comprising water glass, a silica sol and a dispersion of an organic resin was disclosed in laid-opened Japanese patent applications No. 43121/72 and No. 23833/73. However, even when such composite coating liquid is used for a coating composition, it is still impossible to prevent occurrence of efflorescence on the coated surfaces and the defects mentioned above with respect to the coating liquid containing a strong alkali cannot be overcome. What is more disadvantageous is that it is very difficult to maintain an applicable viscosity over a long period of storage. Various composite coating liquids comprising a silica sol and a dispersion of an organic resin have heretofore been proposed, but coating compositions prepared from such composite coating liquids are still insufficient in the adhesion strength and the storage stability for a long period of time.
In general, coating compositions include so-called paints, namely coating compositions for thin coating. In these so-called paints, the coating amount is usually 50 to 600 g/m.sup.2 and it is about 200 g/m.sup.2 in many cases. Accordingly, the problem of sagging is not serious in a coating composition of this type. Further, the solid content is lower than 50% in paints and it is not particularly indispensable to maintain the viscosity at a certain high level for a long period of time. In contrast, in the case of a coating composition for thick coating, one of great objects of which being to hide convexities and concavities on a substrate to be coated, it is important that sagging is effectively prevented and coating composition has such a property that an appropriate pattern can readily be formed on the coating. It is necessary to adjust the coating composition so that a desired pattern can be obtained without sagging while the amount coated is within a range of from 1 kg/m.sup.2 to 20 kg/m.sup.2, generally from 2 to 5 kg/m.sup.2.